The present invention is directed to a clamp for firmly holding and securing a workpiece, or for use as a welder's ground clamp. Clamps are used in multifarious environments, such as for holding wooden furniture parts together for adhesive bonding, for holding metal parts together during the welding process, and the like. The clamp of the invention is in that class defined as spring clamps which provide a pair of clamping or holding jaws, one of which is pivoted to the other and urged toward the other jaw by a spring to secure the workpiece thereat in sandwich-like fashion. These prior art clamps, however, are not easily and readily adaptable to workpieces that vary greatly in size, whereby each clamp is expressly and specifically suitable for only a narrow range of sizes of workpieces. For larger-sized workpieces, larger spring clamps must be used, that cannot accomodate smaller-sized workpieces.